SOI devices are known to potentially have advantages (e.g., reduced junction capacitance, increased radiation hardness) over conventional silicon devices. Several techniques for producing SOI heterostructures and, in particular, for producing Si/SiO.sub.2 /Si heterostructures of the type shown schematically in FIG. 1, are known. Among the known techniques for producing such a heterostructure is the oxygen implantation technique, and this application pertains to this particular technique (sometimes referred to as "Separation by implanted oxygen" [SIMOX]) for forming such a heterostructure. For a review of the SIMOX technique see, for instance, P. L. F. Hemment, Materials Research Society Symposia, Proceedings, Vol. 53, pp. 207-221 (1986).
As generally practiced in the art, the SIMOX technique comprises implanting a sufficiently high dose of oxygen into a silicon body (typically a Si wafer) such that a stoichiometric oxygen-rich region is formed within the body. By this we mean that the oxygen distribution within the silicon body reaches a maximum concentration of at least two oxygen atoms per silicon atom. A typical implantation dose is 2.times.10.sup.18 oxygen atoms/cm.sup.2. The (energy dependent) critical dose .phi..sub.c is the minimum dose at which, for a given implantation energy, a stoichiometric implant results. For instance, .phi..sub.c is about 1.4.times.10.sup.18 cm.sup.-2 at 200 keV.
Prior art stoichiometric implants result in the formation of relatively thick (typically about 0.3 .mu.m or more) layers of SiO.sub.2, with a relatively thin (exemplarily about 0.1 .mu.m) silicon overlayer. For several reasons, it would be desirable to be able to use lower implant doses, and/or to produce SOI wafers having a thinner buried oxide layer. Lower doses are desirable because they generally produce less damage in the Si body, and because they make possible increased throughput. Thinner oxide layers make possible devices requiring lower back bias isolation voltage, as compared to devices using a prior art (thicker) buried oxide layer.
Attempts have been made to obtain these desired results by merely reducing the implant dosage below .phi..sub.c such that a substoichiometric implant region, i.e., an implanted region wherein the maximum concentration of oxygen is everywhere less than two oxygen atoms per silicon atom, is formed. See, for instance, J. Stoemenos et al, Applied Physics Letters, Vol. 48(21), pp. 1470-1472 (1986). These authors report that implantation of a (only slightly) subcritical dose (1.3.times.10.sup.18 cm.sup.-2, 200 keV) of oxygen results in formation of an oxygen-rich layer with dispersed Si islands therein, and that annealing of such a sample at 1150.degree. C. for two hours results in a coarsening of the dispersed Si and formation of SiO.sub.2 precipitates in the Si overlayer near the Si/SiO.sub.2 interface. Such a wafer in general would not be acceptable for device maufacture. Stoemenos et al also report that annealing such a wafer at 1300.degree. C. for 6 hours produces a Si overlayer free of SiO.sub.2 precipitates but produces a buried SiO.sub.2 layer that contains a significant volume of dispersed Si islands. It is known that such islands may act as overlapping floating gates if MOS devices were formed in such SOI wafers. Thus, such prior art SIMOX wafers typically are also not acceptable for device manufacture.
In general, two different situations can be identified. If the silicon substrate is at a relatively low nominal temperature (typically less than about 350.degree. C.), during the subcritical ion implant, appropriate heat treatment can result in formation of a relatively homogeneous thin SiO.sub.2 layer and recrystallization of the silicon overlayer. However, under these conditions the region of the overlayer that is adjacent to the Si/SiO.sub.2 interface is generally heavily twinned, frequently making the heterostructure unsuitable for device fabrication. On the other hand, if during the subcritical implant the substrate is at a relatively high nominal temperature (i.e., typically greater than about 350.degree. C.), subsequent heat treatment generally results in formation of a nonhomogeneous buried SiO.sub.2 layer containing dispersed Si regions, and the silicon overlayer typically also comprises dispersed second phase (SiO.sub.2) regions. In this instance also the method typically does not result in a heterostructure that is useful for device fabrication.
In view of the potential advantages associated with low dose implanatation and with SIMOX heterostructures which have a relatively thin buried SiO.sub.2 layer, a substoichiometric (subcritical) implant technique that can be used to reliably produce a Si/SiO.sub.2 /Si heterostructure in which the buried SiO.sub.2 layer is free of Si islands and in which the Si overlayer has a relatively low defect density and is of device quality would be of considerable significance. This application discloses such a technique.